Various species of chlorine are used in small- and large-scale bleaching, oxidation, and disinfection operations. These operations range from providing a weak sodium hypochlorite solution in a bottle for household whitening and disinfection (liquid bleach solutions, about 5% sodium hypochlorite), to delivering pure chlorine gas to a wastewater treatment plant waste stream. One problem with the use of pure chlorine gas, however, is its high toxicity and risk to workers case of leaks and accidents.
A common approach for large-scale water purification that can be safer than the transportation and subsequent on-site use of chlorine gas is the on-site production of chlorine dioxide. This strong oxidant is used for oxidation to disinfect water flows in drinking water treatment plants and in wastewater treatment plants. As a strong oxidant, chlorine dioxide destroys viruses, bacteria, and other microscopic organisms as it oxidizes compounds having a lower oxidation potential than itself. To maximize its oxidation and disinfection effects, in a water treatment system chlorine dioxide is preferably added after the sedimentation tank or basin.
Chlorine dioxide (ClO2; CASR n 10049-04-4) is a greenish-yellow gas at room temperature that is stable in the dark but unstable in the light. As noted, it is recognized as an extremely powerful biocide, disinfectant agent and oxidizer. As to regulatory allowance of chlorine dioxide in commercial and wastewater and water purification applications, in 1967, the United States Environmental Protection Agency (“EPA”) first registered the liquid form of chlorine dioxide for use as a disinfectant and sanitizer. In 1988, EPA registered chlorine dioxide gas as a sterilant.
Chlorine dioxide kills microorganisms by disrupting transport of nutrients across the cell wall. Chlorine dioxide can be generated in a gas or liquid form and smells like chlorine bleach. However, chlorine dioxide is not to be confused with chlorine gas. They are two distinct chemicals that react differently and produce by-products that also have little in common.
Chlorine dioxide, ClO2, offers the following benefits. First, ClO2 functions via an oxidative rather than chlorinating reaction, the mode of action of chlorine gas. This virtually eliminates the formation of chlorinated organic compounds that are suspected to increase certain cancer risks. Second, ClO2 when generated on site, eliminates the need for site storage of chlorine and/or transportation thereof.
Several types of chlorine dioxide generators are commercially available. Many still utilize gaseous chlorine in their generation process, and while effective, the risk management issues associated with chlorine still remain. Embodiments of the present invention do not use chlorine gas as a reactant. As a result, there is less risk of harm in use of embodiments of the present invention. Further, it is noted that chlorine dioxide gas, is unstable and explosive at pressures over about 40 kPA. Thus the gas form is not routinely safely transported, and instead is produced at the site of use. One system, described in U.S. Pat. No. 6,325,970B1, issued Dec. 4, 2001, uses an in-line system that combines a chlorite, a chlorine donor, and an acid in an in-line system in which chlorine dioxide is formed and introduced directly into a water flow. A typical solution taught by U.S. Pat. No. 6,325,970B1 is a mixture comprising about 10 percent of a 28 percent sodium chlorite solution, about 10 percent of a 12 percent sodium hypochlorite solution, about 1.5 percent of a sodium hydroxide solution, and about 80 percent water. To generate chlorine dioxide, acid is added; the release of chlorine dioxide is stated to be faster with stronger acid solutions. It is stated that the molar ratios of the chlorite and chlorine donor are set such that substantially no gaseous chlorine dioxide is formed. It appears another factor is the relatively low concentrations of the reactants, and the overall reaction conditions. While this approach provides a margin of safety by avoiding the generation of gaseous chlorine dioxide, it is limited to producing relatively low concentrations of chlorine dioxide. For instance, it is stated that a preferred embodiment yields 20,000 to 50,000 parts per million (ppm) of chlorine dioxide before dilution. This is less than five-percent active chlorine, which is very dilute for industrial and municipal bleaching and disinfection operations, respectively.
In addition to U.S. Pat. No. 6,325,970B1, many other approaches are known in the art to produce chlorine dioxide from relatively pure metal chlorite salts, such as magnesium, calcium or sodium chlorites, that are reacted with an acid and/or a chlorine donor. Such approaches may provide yields of chlorine dioxide that range, for instance, from 60 to 98 percent. However, many of the published or patented methods are based on experimentation using, or assuming, high purity (near 100%) of the chlorite or chlorate reactants. Often, when such methods are used with lower purity technical or commercial grades of reactants, the stated yields are considerably lower than the yields stated in such papers and patents. Also, the range of undesired by-products of reactions using the lower purity technical or commercial grades can be greater, and the concentrations of such undesired by-products can be unacceptably high for certain disinfection applications.
Further, it is noted that U.S. Pat. No. 5,061,471 (issued Oct. 29, 1991 to Sundblad and Lovetro), and their prior published application, EPO 88850011.3, disclose a process for continuous production of chlorine dioxide in a cooled reaction vessel subjected to “overpressure.” In contrast to the present invention, the '471 patent discloses a different set of reactants, namely, combining aqueous solutions of an alkali metal chlorate, sulfuric acid, and bubbling in gaseous sulfur dioxide (or, alternately, adding liquid sulfur dioxide). Also, the reaction chamber includes a coil through which a cooling liquid may be passed. It is stated that one advantage of the use of sulfur dioxide is that it reduces chlorine in the reaction vessel.
Many references disclose methods of production of chlorine dioxide. However, these references have not achieved the reliable results and consistent operation of the present invention, using the reactants and conditions of the present invention.
Another aspect of certain embodiments of the present invention is the addition of longer-surviving disinfectant species in combination with chlorine dioxide, for certain applications. Despite its basic effectiveness as an oxidant and disinfectant, chlorine dioxide alone may not provide sufficient disinfection over a sufficiently long time and distance in pipes compared to other chlorine species. There are reports of odor and/or taste issues when using chlorine dioxide as the only chlorine species in a water treatment plant. A possible solution is to add or to co-generate chloramines, which are typically produced as by-products of chlorine gas disinfection, and which are known to have longer term effect as residual chlorine species. With chloramine acting as a secondary disinfectant and chlorine dioxide serving as the primary disinfectant, more effective disinfection and reliable odor and taste removal are achievable.
Given the toxicity and risk inherent in the use of chlorine gas, there is a need to develop a safer and reliable alternative to its use in oxidation and disinfection applications. Given the overestimates of yields and understatement of by-products by known methods of chlorine dioxide production when technical or commercial grades of starting materials are used, there is a need to develop a method that reliably and consistently can utilize technical and commercial grades of chlorine reactants to produce chlorine dioxide at sufficiently high, economical yields with a minimum of undesirable by-products. Finally, given the need in some applications to add chloramines as secondary disinfectants when chlorine dioxide is used as the primary disinfectant, there is a need, at least for some applications, to have an effective means to add or produce a given quantity and type of chloramines in a disinfection system or process.
It also is noted that given updated regulations for EPA Class I wastewater treatment plant effluents, as stated in recent disinfectant byproducts rules that mandate lower levels of trihalomethanes and haloacetic acids, it is expected that the substitution of chlorine dioxide for chlorine disinfection will be favored in order to meet these new rules and related standards. Under such a regulatory environment, the present invention is expected to find many applications in new Class I wastewater treatment plants.
Finally, although chlorine dioxide delivered in solution is relatively safer than chlorine gas, the gas form is more toxic and dangerous. However, because the economics of production favor the generation of the gas, there is a need to develop a system using chlorine dioxide gas in which the chance of leakage or exposure is minimized.
Five references that provide relevant background information about disinfection and chlorine dioxide are “Guidelines for Drinking-Water Quality”, 2nd Edition, World Health Organization, Geneva, “The Chlorine Dioxide Handbook,” by Donald J. Gates, June 1998, AWWA, published as part of the Water Disinfection Series, ANSI/NSF Guideline 61, Weast, R. C., “CRC Handbook of Chemistry and Physics”, 52nd edition, p. D-105, 1971 (no month), and W. J. Masschelein's basic textbook entitled: “Chlorine Dioxide. Chemistry and Environmental Impact of Oxychlorine Compounds”, pp. 112 to 145. 1979 (no month). These references, and all patents and other references cited in this disclosure, and hereby incorporated by reference into this disclosure.
The present invention, described and claimed below, advances the art by providing a reaction chamber, a system, and methods for the production of chlorine dioxide gas for oxidation and disinfection purposes. As described below, it advances the art by meeting the needs stated immediately above.